1. Field of the Invention
The present invention relates to a photosensor. In particular, the present invention relates to a photosensor which include thin film transistors with high field-effect mobility and small characteristic variations and a photoelectric conversion element. Further, the present invention relates to a line sensor or an area sensor in which a plurality of pixels including the photosensor are arranged. Further, the present invention relates to a display device including the photosensor.
2. Description of the Related Art
In recent years, an image sensor including a photoelectric conversion element such as a diode, a CCD, and a CMOS which can read out an electrical signal having image information from an optical signal of image information or text or graphics on paper has been used.
A line sensor and an area sensor are image sensors including a plurality of photoelectric conversion elements. A line sensor includes a plurality of photoelectric conversion elements linearly arranged. An object is scanned with the sensor, whereby an image is taken as an electrical signal. An image scanner can exemplify an apparatus including the line sensor. On the other hand, an area sensor includes a plurality of photoelectric conversion elements arranged in a matrix. An image of an object is projected onto the sensor, whereby the image can be captured as an electrical signal. A video camera and a digital still camera can exemplify an apparatus including the area sensor. In the video camera and the digital still camera, an image of the object is reduced and projected onto the area sensor through an optical system and captured as an electrical signal. Besides, there is a contact area sensor which captures an image of an object such as paper or the like as an electrical signal when the paper or the like is placed in contact with the sensor.
Further, a semiconductor device in which an area sensor is incorporated in a display device so that a display region also serves as an input region has been proposed. A display device which has a function of capturing an image using an area sensor is disclosed in Patent Document 1 and Patent Document 2. Further, Patent Document 3 discloses a display device provided with a sensor for controlling the luminance of a light-emitting element.
The display device incorporating an area sensor can not only capture images but also be used as a so-called touch screen, which detects a pen or finger touching its display region in order to input information. The touch screen is easy to operate, allowing a user to feel as if he/she was operating the touch screen by directly touching a displayed image.
[Patent Document 1] Japanese Patent No. 4112184
[Patent Document 2] Japanese Published Patent Application No. 2001-339640
[Patent Document 3] Japanese Published Patent Application No. 2003-167550